I attended a Ph.D. defense a few weeks ago on the effects of salmon lice (which are copepods, not insects) on their juvenile hosts. The student showed some gory photos and pointed out that for such a little fish, carrying a louse was like a human lugging around a raccoon on his back. Some mites can be just as burdensome, such as this Macrocheles muscaedomesticae (Scopoli) attached to the abdomen of an unfortunate Drosophila hydei Sturtevant.
Females of many species of Macrocheles (Mesostigmata: Macrochelidae) hitch rides on winged insects to move from a place to place, a phenomenon called phoresy. Strictly phoretic organisms do not feed on the host while attached. A great many mite taxa fall into this ‘purely phoretic’ category. Others may facultatively snack on the host while in transit. My colleague Lien Luong has investigated one such mite species, Macrocheles subbadius (Berlese), and its cactus-associated host Drosophila nigrospiracula Patterson & Wheeler. When Lien moved to the University of Alberta it proved difficult to replicate the system, in part because cacti are not common in this part of Alberta. Compost bins are abundant, however, and Lien and her students are investigating the ecological relationship between two compost-associated species, Macrocheles muscaedomesticae and Drosophila hydei. Does M. muscaedomesticae feed on its host while attached, or is it just holding on? One way to test this is to determine whether the mite’s mouthparts pierce the fly’s integument. In this N = 1 sample, the mite just seems to be holding on firmly, probably uncomfortably so from the fly’s point of view.
But by definition, facultative parasites aren’t always parasitic. More mites must be examined, and other lines of evidence followed, such as presence of melanized wounds on hosts after the mites have dropped off, or presence of Drosophila DNA in the guts of the Macrocheles.